For a circuit board for power module, there is conventionally used a bonded substrate made by bonding a ceramic substrate such as an inexpensive aluminum oxide substrate, a highly thermally conductive aluminum nitride substrate, a highly strong silicon nitride substrate, and a metal plate such as a copper plate having a large thermal conductivity by a high melting point metal method using molybdenum (Mo) or tungsten (W), a DBC (Direct Bonding Copper) method using a eutectic reaction of copper and oxygen, an active metal bonding method using an active metal such as titanium (Ti), or the like. A circuit board is configured as a result of patterning the metal plate having been bonded to the ceramic substrate, by means of etching, for example. Among various bonding methods, the active metal bonding method is commonly used since a bonding strength can be heightened.
When a semiconductor device is to be configured by using a ceramic circuit board, a semiconductor chip is mounted on a metal plate such as a copper plate via a solder layer. As one of characteristics demanded of the ceramic circuit board, there can be cited a thermal cycle test (TCT) characteristic. The TCT is a test in which a ceramic circuit board is held for a predetermined period under circumstances of a low temperature, a room temperature, and a high temperature and it is investigated what degree of durability against such thermal change the ceramic circuit board has.
In order to improve the TCT characteristic of the ceramic circuit board, a structure is suggested in which a brazing material layer is protruded from an end portion of a copper plate. In a ceramic/copper circuit board having such a structure, when a TCT one cycle of which is −40° C.×30 minutes→room temperature×10 minutes→125° C.×30 minutes→room temperature×10 minutes is carried out, it is reported that a crack does not occur in a ceramic substrate even after 30 cycles. As a semiconductor chip comes to have a higher power, there is required a ceramic/copper circuit board in which a crack does not occur in a ceramic substrate at a 1000 cycle level in TCT characteristic.
Further, there is suggested a ceramic/copper circuit board whose TCT characteristic is improved by using a silicon nitride substrate as a ceramic substrate and controlling a composition of a brazing material protruded from an end portion of a copper plate. In such a ceramic/copper circuit board, when TCT is carried out under a severer condition of −50° C.×30 minutes→room temperature×10 minutes→155° C.×30 minutes→room temperature×10 minutes being one cycle, it is reported that a crack does not occur in the ceramic substrate at a 5000 cycle level.
A semiconductor chip is being made to have a higher power. Under such a circumstance, it is estimated that an operation temperature of a Si element, which has conventionally been about 100 to 130° C., rises to about 160 to 190° C. Further, it is estimated that an operation temperature of a SiC element becomes as high as 200 to 250° C. In order to cope with the semiconductor chip coming to have a higher power and the operation temperature coming to be higher as above, improvement of TCT characteristic under a severer condition is required of a ceramic/copper circuit board.
As described above, TCT characteristic is improved by using a silicon nitride substrate as a ceramic substrate. On the other hand, a durability of only about 300 to 400 cycles is obtained in a case of an aluminum nitride substrate or an aluminum oxide substrate. The silicon nitride substrate can be strengthened to 600 MPa or more in a three-point bending strength. Though the TCT characteristic can be improved by using the silicon nitride substrate as above, the silicon nitride substrate is generally more expensive compared with the aluminum nitride substrate and the aluminum oxide substrate, and thus a manufacturing cost of a ceramic/copper circuit board is increased. Thus, a ceramic/copper circuit board capable of improving TCT characteristic even in a case of using an aluminum nitride substrate or an aluminum oxide substrate is required.
When a semiconductor chip is to be mounted on a ceramic/copper circuit board, a bonder-mounter device is commonly used. In such a device, a surface of a copper plate is image-recognized to detect a position, the semiconductor chip is positioned in relation to the copper plate whose position has been detected, and thereafter, the semiconductor chip is mounted on the copper plate. Position detection of the copper plate is performed by detecting a position of an end portion of the copper plate by using a detector such as a CCD camera. When a composition of a brazing material protruded from the end portion of the copper plate is controlled in order to improve TCT characteristic, the end portion of the copper plate becomes a gentle inclined surface. With such a gentle inclined surface, a defect such as reduction of a detection accuracy of the copper plate end portion by the detector occurs. If the gentle inclined surface is formed in the copper plate end portion, an area in which the semiconductor chip can be mounted becomes smaller in relation to an area of the copper plate. Thus, constraint on design of a semiconductor becomes large.